Many animals can use differences in timing and loudness of a sound signal between the ears to localize its source, because these disparities vary systematically as the source angle changes. When sound comes from the right, it reaches the right ear before it does the left ear. This is how an interaural time difference is created. The sound may be also louder in the right ear than in the left ear, because the head reduces its loudness by blocking and absorbing the sound. This is how an interaural level difference is created. Mammals including humans and birds use similar methods to measure interaural time differences. Barn owls are particularly suitable for the study of brain mechanisms of sound localization. They use the interaural time difference for localization in the left-right direction and the interaural level difference for the up-down direction. In their auditory space, each direction corresponds to a particular combination of time and level differences. The owl's brain contains a map of auditory space in which each nerve cell is sensitive to particular combinations of time and level differences. Experiments showed that the owl's brain multiplies the time and level differences in order to create the sensitivity to combinations of time and level differences in the direction-sensitive nerve cells. The present project investigates how the multiplication is done in terms of movement of ions across the cell membrane. The response of these nerve cells to time-level differences involves both excitatory and inhibitory input from other nerve cell. The present project will determine where the inhibitory cells are located. The map of auditory space contains many nerve cells, but little is known about how these cells respond in behaving owls. The present project will monitor many cells simultaneously in awake birds. Finally, the first brain site where interaural time differences are detected is the subject of investigation. The question is how single nerve cells can detect microsecond time disparities. The physiological properties of these nerve cells will be studied to answer the question.